Detecting AC Motor Speed 2

[Updraft]

I am interested in detecting motor speed on an AC motor. We do not have the EE expertise so I turn to you in the Eng-Tips forum. We used a meter in the lab to show that there are detectible pulses generated by the motor when we spin the shaft with no power applied. Our question is this; can we take advantage of this to determine the speed of the motor while it is powered and under load?

Ultimately we are interested in being able to determine when the motor or the system needs some sort of attention by sensing the motor speed or some other indicative parameter. Others have proposed using a Hall Effect system, but we are trying to make this as simple as possible.

Thanks in advance!

- - -Updraft

 

[jraef]

What you want is what Skogsgurra already mentioned, an "Active Power" (denoted I_res) monitor. It combines current AND power factor for more reliability. Don't forget, current will change with voltage, so if you are monitoring LOAD, the load will not change with voltage fluctuations. Power factor alone is better than current alone, but it is not very linear and difficult to apply setpoints to. Active Power monitors do the math inside of the relay and allow simple linear setpoints to be applied for limit alarms, which is how I interpret your need here. The Load Controls unit mentioned above is one that works if you need all 3 phases monitored, another is made by Emotron, but if you don't need an analog output and want just high/low setpoint tripping, the simplest and least expensive I know of is this one:

Siemens 3UG4641 I_res monitor

Under the description is a javascript link that says "Illustration for current/cos ? monitoring" and if you click that, it pops up a chart that explains the differences very nicely.

_{Disclaimer: I work for Siemens, but this is not an advertisement. I know this produce very well, but there may be others like it, I just don't know.}

 

[jraef]

By the way, to address your earliest question:
You can monitor speed of the motor based on the pulsations you noticed, but not while it is running. Those pulsations are caused by the rotor bars passing through some residual (or applied) magnetism in the stator windings. There are what are called "standstill" monitors which do that to determine, for safety reasons, whether a disconnected motor is still spinning. Some DC injection brakes also use that method to detect when to turn off the DC injection, but they monitor the unused winding in that case.

 

[itsmoked]

I have a hard time seeing how you are going to watch the long term aging of a motor with current monitoring, or power monitoring.

How does this power monitoring deal with a load like a conveyor, or a mixer, or an elevator, or refrigeration? These loads wander all over the place. They can vary by 70%! These variations don't generally matter even if they are substantially over the motor's rating, just as long as the average value keeps the motor below its designed temperature rise. How does this current monitoring show that the motor has thrown its fan? How does it show that the cooling ports or fins are completely clogged shut? Only the case temperature is going to warn you of this.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Sorry, didn't see where the OP was asking about motor aging. I thought he wanted to know if the belts were slipping (load decrease) or the machine was jammed (load increase).

 

[itsmoked]

Well, I guess we can take it either way...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Let's start over.
Tell us your problem. If you need help with a problem, tell us the problem. You have decided that knowing the motor speed may solve your problem. Yes we can help you determine the motor speed, but this may not be the best solution to your problem, and may not even solve your problem.
The speed of an induction motor varies a little when the load increases or decreases, but not much. Slipping belts or load loss is a serious safety issue in grain elevators. It is checked by a speed switch on the driven equipment, not on the motor. There are other issues that you may wish to monitor, but each should be evaluated and solved independently.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[peebee]

I still say that for the stated purposes, namely "belt breaks...jamming...simple as possible", that current sensing should be just fine.

If a greater accuracy is required, then an external speed sensor would likely be required, eg., to determine if a belt has broken or if the system is simply unloaded, or to distinguish between an overload and a jam.

 

[waross]

Belt slipping may not change the current enough to be a reliable indicator. This may be the most dangerous case, as it may result in molten burning belt material being thrown around, starting fires.
Several very large grain elevators destroyed with loss of life bear testimony to this. Elevators now use speed monitoring on the driven machinery, configured as slip detectors, to alarm belt problems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScumPunk]

Hi, all,
I think that given the small size of some of these motors (1/4HP), sensing the current alone would be impractical. I've noticed several times that the basic current on a small motor appears to FALL as load is applied...there is also a very small difference between unloaded and full load currents. It would be simple to check these conditions in your applications I guess, just put a current clamp on your lead, connect to a DMM and take readings in the various conditions...
Cheers,
Mort

 

[LionelHutz]

I'm with Bill. If you want to detect belt slipping then you most likely need speed indicators on the driven equipment. Especially if the equipment in question will have a varying load. It's impossible to detect between belt slippage and typical low load operation.

Maybe something from

http://www.loadcontrols.com

would work for you.

 

[itsmoked]

I wonder if you could non-contact detect slipping with the temperature of the belt. One non-contact sensor attached to an ambient sensor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

if your worried about belt slippage use a PPI encoder mounted to the bottom of the belt.

http://www.photocraftencoders.com/Wheeled Encoders.htm

compare this to an encoder on the motor, obviously do the math to get the gear changes.

http://www.photocraftencoders.com/Wheeled Encoders.htm